The present invention relates to devices for spot welding of structures constituted by metal elements, in particular motor-vehicle bodies or sub-assemblies thereof, of the known type comprising:
a welding station, provided with automatic spot welding means, PA1 a conveyor line for feeding the welded structures going out of the welding station in sequence downstream thereof, PA1 at least first and second locating gates which are rapidly interchangeable at the welding station and carry devices for clamping the component elements of the structure to be welded in the proper mutual assembling position, PA1 said locating gates being adapted to operate respectively on two different types of structure to be welded, PA1 the locating gate which each time is at the work area in the welding station being further movable transversally to the conveyor line between an operative position, in which the clamping devices carried by said gate locate the components of the structure which is at the welding station in the proper mutual position, and an inoperative position, in which the locating gate enables the welded structure to go out of the welding station, PA1 each locating gate being provided with a self-propelled lower trolley guided on a rail provided along the floor of the welding station, in order to be movable between a work position at the welding station and a waiting position spaced apart therefrom, so that the station has no supporting frames for elements guiding movement of the locating gates.
A device of the above described type has formed the subject of European patent application EP-A-0 642 878 of the same Applicant. The latter has been manufacturing and marketing for years a flexible welding system, identified with the trademark "ROBOGATE", whose basic concept has been originally disclosed for example in U.S. Pat. No. 4,256,947 and in the corresponding German patent No. 28 10 822 and which gave raise with the time to a number of subsequent improvements and variants which also have formed the subject of various patents of the Applicant. The ROBOGATE system has indeed determined a turnaround in the welding technique of motor-vehicle bodies which had been generally used up to the end of the seventies and has replaced the previous equipment used with many car manufacturers all over the world. In a basic version of this system, it comprises two or more pairs of locating gates which are rapidly interchangeable at the work position in the welding station and are to operate on respective types of bodies. The system is able to operate on bodies which are even very different from each other, so that a same line can be used for the production of different models. A further advantage of the ROBOGATE system lies in that it can be adapted with relatively simple and rapid operations and thus with very reduced investments to the production of a new model of body. Another advantage lies in that a uniform quality is insured on all the models of a same type on which the system operates.
The prior art constituted by the ROBOGATE system provides that each body to be welded reaches the welding station in a loosely pre-assembled condition. Indeed, upstream of the ROBOGATE welding station, there are provided one or more hemming stations, at which the various elements constituting the body are loosely assembled with each other by bending connecting tabs. This provisional connection is naturally a loose connection, i.e. it allows small movements of each element with respect to the adjacent elements. Just for this reason, the locating gates provided at the welding station have clamping devices which engage the various parts of the body in order to clamp them in the proper assembling position before welding spots are applied by the robots which usually constitute the welding means provided at the station. Once a number of welding spots has been applied on the body (at one or more ROBOGATE stations) sufficient to provide the body with a stable geometry, the body proceeds towards further stations for completing welding, which do not require the use of clamping devices.
Subsequently to the above mentioned patents, the Applicant has filed further patents relating to various improved versions and developments of the ROBOGATE system. A last one of these improvements has formed the subject of European patent application EP-A-0 642 878 which has been mentioned above. In this document there is disclosed a welding station which has a much less complicated, lighter and more flexible structure with respect to the prior embodiments of the ROBOGATE system. In the most conventional solutions, said locating gates are slidably guided, in order to enable them to be rapidly interchanged at the work position with the gates provided for a different type of body to be welded, on overhead guides directed parallel to the direction of the conveyor line in the welding station, which require the provision of strong fixed supporting frames. These frames, beyond greatly contributing to the cost and the bulk of the welding station, also make difficult for the welding robots to find the space necessary to operate on the body to be welded. In the solution disclosed in European patent application No. EP-A-0 642 878, each locating gate is movable separately from the other gates since it is provided at its lower part with a self-propelled trolley which is guided on a rail provided along the floor of the welding station. Therefore, the locating gates do not require the provision of longitudinal guides which extend throughout the whole length between their work position and their waiting position, and hence do not require the provision of the heavy and bulky supporting frame, constituted by gantry structures connected to each other longitudinally, which was necessary instead in the classic ROBOGATE solutions. Thus, the space available for the welding robots is greatly increased, which enables also, if desired, the number of robots to be increased in order to obtain an increase of the welding spots applied during a determined time interval. The robots themselves may access more easily to all the parts of the body to be welded in order to carry out proper weldings. Furthermore, since each locating gate is movable separately from the other gates, the pitch between each gate and the subsequent gate in the line is free and variable, which provides a great flexibility in design and installation of the plant. Furthermore, the installation of the plant is of a more reduced cost with respect to that of the conventional plants.
The embodiment which is disclosed in the above mentioned European patent application EP-A-0-642 878 relates to a case in which all the components of the structure to be welded, in the specific case of a motor-vehicle body, are brought to the welding station from said conveyor line, which has support means of any type on which the various components of the body are suspended in positions close to each other. When the components of a body to be welded reach the welding station, the locating gates close on these components and locate them in the proper assembling position, in which the robots may carry out the welding spots.
However, the use of locating gates provided at their lower parts with self-propelled trolleys, with the resulting possibility of eliminating the conventional fixed support structure at the welding station, opens to designers the possibility of providing welding stations which operate also according to different criteria, while keeping the same advantages of the solutions which have been described above. This is true both in the case that the structure on which the system must operate is a whole motor-vehicle body, and in the case it is instead only a single body sub-assembly.